There are may industrial, medical, and other applications where high resolution (generally less than 10 micrometer) images of, and measurements of distances, thicknesses, and optical properties of, a biological or other sample are required.
Copending application Ser. No. 07/692,877 mentioned above describes an optical coherence domain reflectometer (OCDR) technique for performing such measurements generally with a single scan in the longitudinal direction. However, there are many applications, including medical applications, where a need exists for such scans to be conducted in two or three dimensions rather than in a single dimension, including transverse directions at a given longitudinal depth, thereby providing multidimensional imaging and measurements. Therefore, a need exists for a means to perform such scanning in at least one transverse direction at a selected longitudinal depth, with the capability of also scanning in the longitudinal direction.
Further, particularly in medical application, it is frequently desirable to provide such scans inside of tubular or other structures such as blood vessels, the bronchial tree of the lungs, the gastrointestinal tract, the genital tract or the urinary tract, using an angioscope or endoscope. In order for such scanning to be performed, a probe must be provided which is capable of being mounted in an endoscope or angioscope for performing internal scans.
While typically a scan would be completed through the full depth range at a given lateral and/or transverse position before repositioning to the next position, this may require scanning of the mirror or other element used for performing longitudinal range or depth scans at a rate higher than the capacity of existing equipment. This is particularly true where the longitudinal scan produces a Doppler shift frequency which affects the interferometric signal frequency and hence the system sensitivity. It is, therefore, desired that such scanning be performed at a constant velocity. However, since very high speed longitudinal scanning at a constant velocity is difficult to achieve, where two or three dimensional scanning is being performed, other scan patterns may be required. Further, in some applications, it may be desirable to perform transverse scanning in one or two dimensions at a selected longitudinal position or depth.
Another problem which becomes particularly serious when transverse scanning is being performed is that the bandwidth of the received signals increases beyond the inherent Doppler frequency shift of the system. In such cases, aliasing (i.e. variations in image intensity) may occur. It is, therefore, desirable that a technique be provided to enhance resolution by eliminating or averaging out such intensity variations.
Another problem with the prior system is that, if scanning is to be conducted over an extended depth range, a smaller numerical aperture must be used so as to extend the depth of focus. However, this reduces lateral resolution and the received optical signal power throughout the range. A need, therefore, exists for a technique which permits the use of a large numerical aperture over an extended depth range within a sample.
Further, some of the problems described which result from performing longitudinal scanning by mechanically moving a mirror or other element may be overcome by performing this scan electronically, for example by varying the optical frequency or amplitude of the light incident from the light source. However, for certain applications, for example imaging a dynamic biological sample such as the eye, the scanning speed required to do three-dimensional scanning may be such that a parallel scanning technique may be preferable or may be required.